Honeycomb structures are used as, for example, a catalyst carrier for an internal combustion engine, boiler, chemical reactor, fuel cell reformer, etc. and a filter for trapping particles presenting in an exhaust gas, particularly particles emitted from a diesel engine (this filter is hereinafter referred to as DPF).
This honeycomb structure is aimed to have a higher porosity, thinner partition walls, and a larger size in order to reduce a pressure loss and to enhance a treatment capability, and this direction degradates strength and dimensional precision of the honeycomb structure. Therefore, some kind of reinforcement is required depending on the cases.
To solve the problem, the method of improving the dimensional precision has been proposed in J-P-B-2604876, and the proposed method comprises that a ceramic honeycomb fired body is prepared, a peripheral edge portion of the ceramic honeycomb fired body is removed, and an outer wall portion is formed on the outer peripheral surface of the fired body. A honeycomb structure having an outer shell layer formed in the same manner as described above is disclosed in J-P-B-2613729 which further discloses that the outer shell layer is made of cordierite and colloidal silica or the like.
The honeycomb structure is often held in a can via a holding member, and connected to an exhaust line of an internal combustion engine or the like. When the structure is mounted on an automobile or the like, there is a risk that a position of the honeycomb structure in the can deviates by strong vibration or an exhaust gas pressure. When the position deviates, there is also a risk that the honeycomb structure does not sufficiently function.
To solve the problem, a ceramic honeycomb structure disclosed in J-P-A-7-4534 is characterized by that a stepped portion is disposed by projecting or denting at least a portion of an outer wall, and the surface of a projecting or denting portion is rougher than another outer surface. A canning structure of the ceramic honeycomb structure is described in JP-A-2000-70545 in which the honeycomb structure having concave/convex portions on the outer peripheral surface is held in a tubular casing. It is also described that a surface roughness Ra of a concave/convex surface is preferably 0.1 to 0.5 mm.
However, when the surface roughness is excessively large, the strength of the corresponding portion sometimes drops. In the above-described technique, a specific process or a specific device is required for making the concave/convex portion, thus a production process is complicated.